Images of astronomical objects produced by ground-based telescopes are limited by degrading effects of turbulent atmosphere. Light from distant objects arrives at Earth as plane waves. In the absence of atmosphere, the theoretical angular resolution of a telescope is limited only by diffraction. Such diffraction is measurable by noting the observing wavelength (λ) and dividing the observing wavelength by the size of the telescope's primary mirror or aperture (D).
However, in reality, one must account for atmosphere. The atmosphere contains cells of air at different temperatures with resulting different indices of refraction. The presence of such atmosphere causes the light to become non-planar. This causes the ground-based telescopes, when they try to focus on these light waves, to obtain images which appear distorted and blurry. Such images also change as a function of time as the atmosphere the light waves travel through also change over time.
In order to compensate for distortions caused by the atmosphere, laser adaptive optics systems have been used. Such systems are capable of measuring the atmospheric distortions induced onto a laser projected into the sky and then apply compensating effects accordingly to the light received by the ground-based telescope since the light would have similar distortions as the laser.
Laser adaptive optics systems can compensate for effects of atmospheric turbulence. A high-powered laser is projected in the direction the telescope is pointed and is used as a probe of the atmosphere. A tiny fraction of the laser light from the high-power laser returns back towards the telescope which has similar non-planar optical distortions as the light being observed. Internal measurements and calculations done by the laser adaptive optics system on the shape of the laser light received can then be used to shape the incoming light waves being observed to be flat (planar) again.